Brief description

Cyanotype is a slow-reacting, non-toxic chemical photographic printing process that produces a Prussian blue monochrome image. Historically used for reproducing complex engineering drawings, maps, and botanical specimens, it represents an extremely accessible, low-tech way of duplicating documents and records without advanced camera equipment.

Use / Function

Primary use: Document replication and architectural/engineering drawing duplication (origin of the term “blueprints”).
Secondary uses: Creating botanical registers, mapping, and artistic printing.
Scale: Small-scale local operations (offices, schools, workshops).

Operating principle

The process is based on the light-sensitivity of ferric (iron III) salts, which are reduced by ultraviolet (UV) light to ferrous (iron II) state, reacting to form insoluble Prussian blue dye:

Sensitization: A liquid mixture of two simple chemical salts—Ferric Ammonium Citrate and Potassium Ferricyanide—is prepared in water and applied to paper. In its unexposed state, this mixture is water-soluble and light-sensitive.
UV Exposure: A negative or physical object (like a leaf or a drawing on translucent paper) is placed on the sensitized paper and exposed to sunlight (UV radiation).
Chemical Reduction: Where UV light hits the paper, the ferric iron is reduced to ferrous iron. This newly reduced iron reacts immediately with the potassium ferricyanide to form ferric ferrocyanide—an insoluble pigment known as Prussian blue.
Washing (Development): The paper is washed in pure Water. The unexposed, soluble chemicals wash away completely, leaving the white of the paper underneath. The exposed, insoluble Prussian blue remains permanently bound to the paper fibers.

How to create it

1. Preparing the Chemical Solutions

Solution A: Dissolve 25 grams of Ferric Ammonium Citrate in 100 ml of Water.
Solution B: Dissolve 10 grams of Potassium Ferricyanide in 100 ml of Water.
Note: Keep these solutions separate and stored in dark bottles in a cool, dark place. They can last for months.

2. Sensitizing the Paper

In a dimly lit room (avoiding sunlight and UV light; standard low-wattage yellow or incandescent light is safe), mix equal parts of Solution A and Solution B.
Using a brush or sponge, coat a sheet of sturdy Paper (such as watercolor paper) evenly with the mixed solution.
Let the paper dry completely in total darkness. Once dry, the paper will have a yellow-green tint and is ready to use.

3. Exposing and Developing

Place the drawing or object you wish to copy directly on top of the sensitized paper. To ensure sharp lines, press them flat together under a sheet of clear Glass.
Expose the assembly to direct sunlight. Exposure times range from 2 to 15 minutes depending on the intensity of the sun. The exposed areas of the paper will turn from yellow-green to a dull bronze-grey color.
Remove the paper from the sun, discard the glass and the object, and immediately submerge the paper in a tray of running Water for about 5 minutes.
The yellow unexposed chemicals will wash away, and the blue color will deepen.
Hang the paper to dry. As it dries, oxidation will fully mature the image into a deep, rich Prussian blue.

Materials needed

Sensitizing Chemicals: Ferric Ammonium Citrate and Potassium Ferricyanide.
Base Material: High-quality, acid-free Paper (or natural Fabric like Cotton).
Solvent: Clean Water.
Tools: Measuring containers, mixing cups, soft brush, a sheet of clear Glass, and a flat backing board.

Variants and improvements

Toning: The finished blue print can be chemically altered (toned) by soaking it in weak alkaline solutions (like Soda or ammonia) to bleach it, and then in tannic acid (from black tea or oak galls) to turn the image to deep brown or black.
Blueprint Machine (Sun-printing): Using industrial UV lamps and rollers to rapidly duplicate technical drawings on long rolls of pre-sensitized paper.

Limits and risks

Alkali Sensitivity: Prussian blue is sensitive to alkaline environments. Washing the prints in alkaline water, or mounting them on alkaline cardboard, will cause the image to fade.
Fading in Strong Light: Over time, prints may fade if left in direct sunlight continuously, though they often partially recover their depth of color if placed in the dark.
Chemical Handling: While relatively non-toxic compared to other historical photography processes (which used mercury, silver, or cyanide), the dry chemical powders should not be inhaled or ingested.